Oriented polyester film, particularly biaxially oriented film composed of polyethylene terephthalate (PET), has been widely used as a packaging material or as a base for microfilm, reprographic films, proofing films and the like. It has good optical clarity and toughness which renders it extremely suitable for such applications.
Polyester films have recently found expanded applications as transparent and translucent layers applied to solid substrates and particularly to transparent substrates. Thus, such films may be applied to windows and viewing or lighting media to control the transparency thereof. Another more recent application for polyester film involved its use as an antilacerative layer in the construction of automotive glazing products such as windshields, rear windows, sun roof panels and the like, and in architectural safety and solar glass. One of the simpler automotive glazing products may comprise a flexible interlayer of polyvinylbutyral sandwiched between glass on one side and a high modulus film, for example polyester film, on the opposite side.
Polyvinylbutyral layers are used in sandwich automotive glass construction to prevent breaking of the glass in the case of impact (so called safety glass), whereas the role of the polyester film positioned on the interior glazing surfaces is to prevent skin lacerations caused by hard impact of the head or body with automotive glass causing the glass to shatter, as a consequence of an automobile accident. Constructions of this type and techniques for applying the polyester film to automotive glass surfaces are disclosed in U.S. copending application Ser. No. 07/299,583, filed Jan. 23, 1989, now U.S. Pat. No. 4,973,364, which is a divisional of application Ser. No. 148,818, filed Jan. 27, 1988 (commonly assigned), now abandoned, the disclosure of which is incorporated herein by reference.
The above referenced copending application teaches the use of a layer of plasticized polyvinylbutyral (PVB) as an adhesive layer to facilitate adhesion of the polyester film to the glass. While PVB adheres very well to glass, the adhesion between it and polyester film is not as great, particularly at subfreezing temperatures. Corona discharge treatment and flame treatment have been used to improve adhesion between PVB sheet and polyester film as disclosed in U.S. Pat. No. 3,900,673. Coatings of gelatin and PVDC and acrylic latex coatings have also improved adhesion between polyester film and PVB. However, none of these methods of imparting adhesion improve adhesion at the subfreezing temperatures to which windows in automobiles and buildings are commonly subjected. It is believed that any improvement of the adhesion of the polyester film to the PVB improves the function of the polyester film as an antilacerative shield.
Numerous primer coatings are known in the prior art for the application to the surface of polyester film to improve the adhesion of the film to various materials. Examples of such coatings include compositions based on vinylidene chloride polymers (U.S. Pat. No. 2,698,240), thermosetting acrylic or methacrylic polymers (U.S. Pat. No. 3,819,773) and like materials. Certain water dispersible copolyesters have also been disclosed as adhesives for laminating sheets of polyester film to one another or to sheets of nylon film, as taught in U.S. Pat. Nos. 3,563,942 and No. 3,779,993. Solvent based polyurethane adhesives have also been used to promote adhesion, but with attendant problems of solvent emissions and safety.
The use of silane coupling agents to improve the bonding properties of polyethylene sheets and polyester sheets is known. For example, polyester sheets and polyethylene sheets primed with either vinyltrimethoxy silane or chloropropyltrimethoxy silane have been successfully laminated using a hot melt adhesive such as an ethylene/vinyl acetate terpolymer or an elastomeric polyester as disclosed by E. Plueddemann, "Bonding Through Coupling Agents", Plenum Press, New York, 1985. In addition, the use of N-2-aminoethyl-3-aminopropyltrimethoxy silane (sold under the trademark Z-6020 by Dow Corning) as a primer coating to enhance the adhesion of an ionomer resin (salt of a copolymer of ethylene and methacrylic acid) to glass and to polycarbonate sheets is disclosed in U.S. Pat. No. 4,663,228. Related silanes such as N-3-aminopropyltrialkoxy silanes are known to enhance the adhesion between polyurethane films and glass substrates, as disclosed in published European Patent Application 171,917.
None of these prior art disclosures, however, addresses the problem of the maintenance of good adhesion between polyester film and polyvinylbutyral or other polymer films at subfreezing temperatures where many primer materials fail.
Accordingly, it is an object of this invention to provide an oriented, self supporting film material having enhanced adhesion when laminated with other polymer films.
A further object is to provide a primer coating for polyester film which enhances adhesion between polyester and polyvinylbutyral films and imparts resistance to delamination even at subfreezing temperatures.